Marathon is a super island because of the excellent sunset viewing locations. This is the number one choice because of the Seven Mile Bridge backdrop. Music is played each afternoon and Sunday is big pool party blast. What a super restaurant, a place that never gets old. It is a lovely place for those big fancy frozen cocktails!
Any place that I can have cold-from-freezer-beer and see the light show is good for me. I think the best spot for sunset is the famous Seven Mile Bridge. You have to at least once go out on the bridge to see the show. It is So Historic and the sun sets straight down towards Big Pine Key. Bring your camera to all the sunset events.